A vibrating micro-electro-mechanical-system (MEMS) gyroscope is one application of a mechanical resonance system and is often used where an angular rotation rate is to be measured. A vibrating MEMS gyroscope includes a movable gyroscope mass (sometimes referred to as a proof mass) that is connected by springs to a substrate. A drive force applied to the proof mass provokes and maintains a constant linear momentum of the proof-mass along a driving position axis, which is needed to generate a Coriolis force ‘Fc’. Coriolis effect is based on conservation of momentum, whereby the Coriolis force ‘Fc’ is proportional to the product of the proof-mass ‘m’, the input rate ‘Ω’, the proof mass velocity ‘v’, and the proof mass's angular rate of rotation perpendicular to the direction of movement of the proof mass. The Coriolis force acting on the proof mass, in the presence of an angular rotation, can be induced using a capacitive force by applying a voltage to capacitor plates of a drive actuation unit. In response to the induced force, the proof mass is moved.
An induced drive force can be supplied and controlled using a drive actuation unit, a drive measurement unit and associated circuitry, which in combination is sometimes referred to as a drive-mode oscillator. The drive actuation unit typically includes a capacitive coupling along the driving position axis between a capacitor plate on the substrate and an opposite capacitor plate on the proof mass.
The drive measurement unit includes a similar pair of capacitor plates. The capacitance between the capacitor plates of the drive measurement unit can be measured and is related to a displacement of the proof mass along a sensing position axis that is perpendicular to the driving position axis. Measurement of the displacement of the proof mass along the sensing position axis can be used to obtain a measure of the Coriolis force and thus a measure of the angular rate of rotation.
A sense measurement unit is also sometimes provided, which, similar to the drive measurement unit, can include a capacitive coupling along the sensing position axis between a sense capacitor plate on the substrate and an opposite sense capacitor plate on the movable proof mass. The sense measurement unit can measure any induced sinusoidal Coriolis force due to a combination of the drive oscillation and any angular rate input. The capacitance between the sense capacitor plates of the sense measurement unit is measured as a sense measurement signal and forms an indication of the displacement of the proof mass along the sensing position axis.
FIG. 1 illustrates a series of drive activation waveforms 100. A first drive activation waveform 110 represents an ideal case, whereby the displacement of the proof-mass is an oscillation along the drive position axis, as illustrated. A second drive activation waveform 170 represents a situation when an angular rate is applied. Here, a displacement is measured on the sense position axis, where the measured displacement is proportional to the Coriolis force. A third drive activation waveform 140 represents the effect of a non-ideal mechanical manufacturing process, or an effect introduced by external stress, whereby the drive proof-mass is forced to not oscillate exactly along the drive position axis. In addition, in this scenario, the drive proof-mass generates a signal along the sense position axis. This additional (undesired) signal waveform is often referred to as a ‘quadrature error’ as the signal waveform is 90° phase shifted from a measurement signal waveform in the ideal case. Thus, the quadrature error of the additional signal is proportional to the displacement of the drive mass, whereas the Coriolis force is proportional to the velocity of the drive mass.
U.S. Pat. No. 7,290,435 B2 describes a way to compensate for mechanical quadrature errors by determining a digital code at a production stage, storing the digital code in a non-volatile memory in a one-time programmable (OTP) manner and using the digital code to set an amplitude of a quadrature error compensating signal. Hence, the solution proposed in U.S. Pat. No. 7,290,435 suffers from practical limitations when applied in the field, particularly in that a quadrature error compensating signal is only identified during the production stage of the MEMS gyroscope.